* GTest will be downloaded as external project if needed

* Added readme.

if compiler num is 0 this is not always an error,
as that  can also happen, when having only a objectlib
as input and no other source files.

Interface libraries added since CMake 3.0 version have INTERFACE_SOURCES property instead of SOURCES so it leads to error which is unclear and prevents project from compiling. Warning would be enough it this case.

Instead of printing the same message over and over again, if a target can't use
a specific sanitizer, the message will be printed only once for the first
affected target now.

In some special cases static linking the sanitizers won't work, and you have to
preload libasan. To find the right libasan depending on the application to be
executed, a wrapper script was added. The location of the wrapper script can be
accessed via ASan_WRAPPER variable.

If you like to preload a sanitized library in front of an application, it will
fail, because it is not the first in the library list anymore. This behaviour
is gcc specific - clang will static link the sanitizers instead. To get the
same for gcc, the new SANITIZE_LINK_STATIC flag will link the sanitizers
static in gcc environments.

Note: The preloaded executable must be sanitized, too! Otherwise ASan will
      print an error message!

Due optimizing the code is optional for using the sanitizers, the user should
set an optimisation level by his choice on his own.

Some sanitizers are not compatible with other sanitizers. Added some checks to
avoid these combinations at configuration time.

If a compiler does not support sanitizing, it will not be checked a second time.

According to the MemorySanitizer documentation, it is only compatible with 64bit
Linux at the moment.

According to the ThreadSanitizer documentation, it is only compatible with 64bit
Linux at the moment.

* Added a new module FindSanitizers with new function add_sanitizers to combine
  all sanitizers in one function instead of adding each one by one.
* Code of FindASan.cmake was outdourced into helper functions, so that the code
  may be used by other sanitizer modules, too.
* AddressSanitizer will be used with -O1 now to get a better performance.

* removed build type ASAN. Targets should define ASan usage by sanitize_address
  function
* compiler flags will be searched by compiler instead of per language
* FindASan warns, if a target can't be sanitized because of incompatible
  compilers
* added some helper functions

Thus C can be mixed with Fortran and the objects can be build with different
compilers, not both may support AddressSanitizer.

Instead of fix checking for C and CXX compilers, all enabled languages will be
tested now.

Older CMake versions are not capable of continue in foreach functions.